Long-term changes in net radiation and its components above a pine forest and a grass surface in Germany

Abstract

Long‐term measurements (1974–1993 and 1996, respectively) of the net radiation (Q), global radiation (G), reflected global radiation (R), long‐wave atmospheric radiation (A) and thermal radiation (E) of a pine forest in Southern Germany (index p) and of a grass surface in Northern Germany (index g) are compared. The influence of changes in surface properties is discussed. There are, in the case of the pine stand, forest growth and forest management and in the case of the grass surface, the shifting of the site from a climatic garden to a horizontal roof. Both series of radiant fluxes are analyzed with respect to the influences of the weather (cloudiness, heat advection). To eliminate the different influence of the solar radiation of the two sites, it is necessary to normalize by means of the global radiation G, yielding the radiation efficiency Q/G, the albedo R/G=α and the normalized long‐wave net radiation (A+E)/G. Furthermore, the long‐term mean values and the long‐term trend of yearly mean values are discussed and, moreover, a comparison is made of individual monthly values. Qp is twice as large as Qg. The reason for this is the higher values of G and A above the pine forest and half values of αp compared to αg. Ep is only a little greater than Eg. The time series of the radiation fluxes show the following trends: Qp declines continuously despite a slight increase of Gp. This is mainly due to the long‐wave radiation fluxes. The net radiation of the grass surface Qg shows noticeably lower values after the merging of the site. This phenomenon is also dominated by the long‐wave radiation processes. Although the properties of both site surfaces alter, Ep and Eg remain relatively stable. Ap and Ag show a remarkable decrease however. The reason for this is to be found in a modification of the heat advection, showing a more pronounced impact on the more continentally exposed site (pine forest). Compared to αg, αp shows only a small variability. The changes of αp can be easily explained as resulting from the influence on the albedo of alterations in cloudiness and the changes in the water storage in stand and soil produced by varying weather conditions. The influence of the forest management is only small and short‐termed. αg reacted with a leap on site merging, followed by a continuous drop. The drop of the grass surface albedo αg on top of the observatory is probably related to the gradual development of the newly sown lawn there and the resultant change in the specific hydrological balance of the soil. A decreasing radiation efficiency Q/G at both sites is observed, which is principally due to an increase in −(A+E)/G. The investigation discovered changes in the long‐wave radiation processes. Its influence in Q should lead to more attention to the long‐wave radiation processes in routine monitoring of radiation. Copyright © 1999 Royal Meteorological Society

Net radiation balance for two forested slopes on opposite sides of a valley

“…The influence caused by the radiative fluxes could be quantified. The differences in K# p between both slopes found in this study were comparable to the differences Kessler and Jaeger (1999) found between two sites in northern and southern Germany about 600 km apart. Thus it appears that the orientation of the slopes has a large influence on the microclimate and hence the growing conditions of the forest vegetation.…”

Comparative analysis of albedo and surface energy balance of a grassland site and an adjacent Scots pine forest

“…Since the weather conditions were almost equal at both sites, different optical features of both land-use types must have been the main cause of this difference. The more pronounced surface structure of the Scots pine at the adjacent Bremgarten weather station forest led to stronger multiple reflection and scattering of the downward short-wave radiation than at the grassland site (Wicke & Bernhofer 1996, Kessler & Jaeger 1999. As a consequence of its lower albedo, the Scots pine forest had a corresponding higher net short-wave radiation than the grassland site (Rost 2004).…”

“…Wicke & Bernhofer (1996) analysed the energetic behaviour of the 2 sites in southwest Germany that are investigated in this study, but only on the basis of an intensive 2 wk measurement campaign characterised by clear-sky weather. Kessler & Jaeger (1999) compared the radiative fluxes between the forest site of this study and a grassland site near Hamburg (in northern Germany). However, sites which are far away from each other are influenced by different synoptic conditions.…”

Long-term comparisons of net radiation calculation schemes

“…In most countries measured R n is available only from well equipped weather stations at research sites, as net radiometers are delicate instruments that require regular maintenance and calibration. Therefore, R n measurements of high temporal and spatial resolution are scarce (Dong et al 1992;Allen 1996;Llasat and Snyder 1998;Kessler and Jaeger 1999;Alados et al 2003). Hence, in lieu of measured R n , calculated values of R n are commonly used (Doorenbos and Pruitt 1977;FAO 1990;Allen et al 1998).…”